Solar Exploration

Mercury is the closest planet to the Sun and the smallest in the solar system, barely larger than Earth's Moon. Named after the fleet-footed Roman messenger god, it earns the title honestly — it completes an orbit in just 88 Earth days, moving faster around the Sun than any other planet. Yet a single day on Mercury, from sunrise to sunrise, lasts roughly 176 Earth days, because its slow three-times-per-two-orbits rotation means the Sun crawls unusually slowly across its sky.

Its surface looks deceptively like the Moon's: a heavily cratered, grey landscape built up over billions of years of impacts with almost nothing to erode or resurface it. But Mercury has its own signature feature the Moon lacks — long, cliff-like ridges called 'rupes' that formed as the planet's iron core cooled and shrank, wrinkling the crust like a drying apple.

With virtually no atmosphere to hold in heat or block radiation, Mercury experiences the most extreme temperature swings of any planet: surface temperatures soar past 430°C in direct sunlight and plunge to -180°C at night or inside permanently shadowed polar craters. Remarkably, those shadowed craters near the poles are cold enough to harbor water ice, confirmed by radar and later by NASA's MESSENGER spacecraft — ice surviving within a stone's throw of the hottest place in the solar system.